anv/cmd_buffer: never shrink the push constant buffer size

If we have to re-emit push constant data, we need to re-emit all
of it.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
CC: <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 198a7222)